AWS Lambda

AWS Lambda Serverless Development

Design, build, deploy, and debug serverless applications with AWS serverless services. This skill provides access to serverless development guidance through the AWS Serverless MCP Server, helping you to build production-ready serverless applications with best practices built-in.

Use SAM CLI for project initialization and deployment, Lambda Web Adapter for web applications, or Event Source Mappings for event-driven architectures. AWS handles infrastructure provisioning, scaling, and monitoring automatically.

Key capabilities:

• SAM CLI Integration: Initialize, build, deploy, and test serverless applications
• Web Application Deployment: Deploy full-stack applications with Lambda Web Adapter
• Event Source Mappings: Configure Lambda triggers for DynamoDB, Kinesis, SQS, Kafka
• Lambda durable functions: Resilient multi-step applications with checkpointing — see the durable-functions skill for guidance
• Schema Management: Type-safe EventBridge integration with schema registry
• Observability: CloudWatch logs, metrics, and X-Ray tracing
• Performance Optimization: Right-sizing, cost optimization, and troubleshooting

When to Load Reference Files

Load the appropriate reference file based on what the user is working on:

• Getting started, what to build, project type decision, or working with existing projects -> see references/getting-started.md
• SAM, CDK, deployment, IaC templates, CDK constructs, or CI/CD pipelines -> see the aws-serverless-deployment skill (separate skill in this plugin)
• Web app deployment, Lambda Web Adapter, API endpoints, CORS, authentication, custom domains, or sam local start-api -> see references/web-app-deployment.md
• Event sources, DynamoDB Streams, Kinesis, SQS, Kafka, S3 notifications, or SNS -> see references/event-sources.md
• EventBridge, event bus, event patterns, event design, Pipes, or schema registry -> see references/event-driven-architecture.md
• Durable functions, checkpointing, replay model, saga pattern, or long-running Lambda workflows -> see the durable-functions skill (separate skill in this plugin with full SDK reference, testing, and deployment guides)
• Orchestration, workflows, or Durable Functions vs Step Functions -> see references/orchestration-and-workflows.md
• Step Functions, ASL, state machines, JSONata, Distributed Map, or SDK integrations -> see references/step-functions.md
• Step Functions testing, TestState API, mocking service integrations, or state machine unit tests -> see references/step-functions-testing.md
• Observability, logging, tracing, metrics, alarms, or dashboards -> see references/observability.md
• Optimization, cold starts, memory tuning, cost, or streaming -> see references/optimization.md
• Powertools, idempotency, feature flags, parameters, parser, batch processing, or data masking -> see references/powertools.md
• Troubleshooting, errors, debugging, or deployment failures -> see references/troubleshooting.md

Best Practices

Project Setup

• Do: Use sam_init or cdk init with an appropriate template for your use case
• Do: Set global defaults for timeout, memory, runtime, and tracing (Globals in SAM, construct props in CDK)
• Do: Use AWS Lambda Powertools for structured logging, tracing, metrics (EMF), idempotency, and batch processing — available for Python, TypeScript, Java, and .NET
• Don't: Copy-paste templates from the internet without understanding the resource configuration
• Don't: Use the same memory and timeout values for all functions regardless of workload

Security

• Do: Follow least-privilege IAM policies scoped to specific resources and actions
• Do: Use secure_esm_* tools to generate correct IAM policies for event source mappings
• Do: Store secrets in AWS Secrets Manager or SSM Parameter Store, never in environment variables
• Do: Use VPC endpoints instead of NAT Gateways for AWS service access when possible
• Do: Enable Amazon GuardDuty Lambda Protection to monitor function network activity for threats (cryptocurrency mining, data exfiltration, C2 callbacks)
• Don't: Use wildcard (*) resource ARNs or actions in IAM policies
• Don't: Hardcode credentials or secrets in application code or templates
• Don't: Store user data or sensitive information in module-level variables — execution environments can be reused across different callers

Idempotency

• Do: Write idempotent function code — Lambda delivers events at least once, so duplicate invocations must be safe
• Do: Use the AWS Lambda Powertools Idempotency utility (backed by DynamoDB) for critical operations
• Do: Validate and deduplicate events at the start of the handler before performing side effects
• Don't: Assume an event will only ever be processed once

Packaging & Dependencies

• Do: Remember that attaching a public or cross-account Lambda layer requires lambda:GetLayerVersion on that layer ARN for the DEPLOYING identity (not just the function role). Restricted CI/sandbox roles frequently lack this and fail at CreateFunction with AccessDeniedException.
• Do: In locked-down environments, vendor dependencies into the deployment package (e.g. pip install -r requirements.txt -t <build_dir>, then zip deps + source together) instead of referencing a foreign-account layer ARN — the package becomes self-contained and independent of the deployer's cross-account permissions. (Alternatively, copy the layer into the target account.)
• Do: boto3/botocore are already in the managed runtime — don't vendor them (wasted size); vendor only the extra libs (e.g. Powertools, jsonschema, PyYAML).
• Do: When you bundle non-code assets (JSON schemas, config, templates) into the package, resolve their path at runtime from a known base (e.g. an env var pointing at /var/task/<dir>, or os.path.dirname(__file__)), not from a build-time relative path that won't exist in the runtime layout.
• Don't: Assume a layer that works interactively will deploy in CI — the deploy principal's permissions differ from yours.

For topic-specific best practices, see the dedicated guide files in the reference table above.

Lambda Limits Quick Reference

Limits that developers commonly hit:

Check Service Quotas for your account limits: aws lambda get-account-settings

Troubleshooting Quick Reference

For detailed troubleshooting, see references/troubleshooting.md.

Configuration

AWS CLI Setup

This skill requires that AWS credentials are configured on the host machine:

Verify access: Run aws sts get-caller-identity to confirm credentials are valid

SAM CLI Setup

1. Install SAM CLI: Follow the SAM CLI installation guide
2. Verify: Run sam --version

Container Runtime Setup

1. Install a Docker compatible container runtime: Required for sam_local_invoke and container-based builds
2. Verify: Use an appropriate command such as docker --version or finch --version

MCP Server Configuration

Write access is enabled by default. The plugin ships with --allow-write in .mcp.json, so the MCP server can create projects, generate IaC, and deploy on behalf of the user.

Access to sensitive data (like Lambda and API Gateway logs) is not enabled by default. To grant it, add --allow-sensitive-data-access to .mcp.json.

SAM Template Validation Hook

This plugin includes a PostToolUse hook that runs sam validate automatically after any edit to template.yaml or template.yml. If validation fails, the error is returned as a system message so you can fix it immediately. The hook requires SAM CLI and jq to be installed; if either is missing, validation is skipped with a system message. Users can disable it via /hooks.

Verify: Run jq --version

Language selection

Default: TypeScript

Override syntax:

• "use Python" → Generate Python code
• "use JavaScript" → Generate JavaScript code

When not specified, ALWAYS use TypeScript

IaC framework selection

Default: CDK

Override syntax:

• "use CloudFormation" → Generate YAML templates
• "use SAM" → Generate YAML templates

When not specified, ALWAYS use CDK

Serverless MCP Server Unavailable

• Inform user: "AWS Serverless MCP not responding"
• Ask: "Proceed without MCP support?"
• DO NOT continue without user confirmation

Resources

• AWS SAM Documentation
• AWS Lambda Documentation
• AWS Lambda Powertools
• AWS CDK Documentation
• AWS Serverless MCP Server